A parametric oscillator is a driven harmonic oscillator in which the oscillations are driven by varying some parameter of the system at some frequency, typically different from the natural frequency of the oscillator. A parametric oscillator can amplify a signal as a parametric amplifier, such as when operated at pump levels below oscillation. A typical parametric amplifier can exhibit much lower noise than an ordinary amplifier based on a gain device like a transistor or vacuum tube based on varying a reactance instead of resistance. An example of a parametric oscillator is a varactor parametric amplifier that operates as a low-noise amplifier in the radio and microwave frequency range. A varactor parametric amplifier can provide non-linear adjustment to the reactance of a given microwave signal via a varactor diode. Applications that require a parametric amplifier can require both high bandwidth and high gain, which can sometimes result in trade-offs in a given parametric amplifier design.